1. Technical Field of the Disclosure
One or more embodiments of the present disclosure relate to the operation of the Physical and Link Layer design of systems such as that of 3GPP's Long Term Evolution (LTE). Specifically, the one or more embodiments focus on the Physical (PHY) and Link Layer design of systems such as 3GPP's Long Term Evolution (LTE). A design according to one or more of the embodiments uses a hybrid Device to UE (D2UE) and Macro to UE (Macro2UE) architecture wherein some functions are maintained by the Macro2UE link and others are supported by the D2UE link.
2. Background Art
One possible way to increase capacity in a wireless network is to increase the density (number of devices per unit area) of deployed base-stations or remote antenna units. Hereinafter, such deployed base station or remote antenna unit is called “small cell unit”. If the density of the small-cell units increases, the cell capacity increases due to frequency reuse effects. However, there are some difficulties that come with increasing the deployment density, especially if such small cell units must be able to operate as conventional base stations on their own.
For example, as the deployment density increases, the number of handovers increases because the user equipment changes the serving unit (base station) frequently. As a result, quality of connectivity/mobility performance is expected to be degraded.
One possible way to improve the connectivity and mobility is carrier aggregation. That is, conventional carrier aggregation operations of the Macro base station and such small cell units can achieve high-quality interworking, because the Macro2UE link can be maintained while UE communicates with small-cell units. As a result, network operators can achieve the same quality of connectivity/mobility as the conventional Macro network.
However, the conventional carrier aggregation operations need to be operated under the single Macro base station. That is, such small-cell units have to be remote radio heads or remote antennas which are perfectly controlled by the Macro base station, i.e. cells wherein small-cell units provide radio communication services must belong to the Macro base station. From a user data point of view, multiple component carriers of the conventional carrier aggregation operations are not visible to the packet data convergence protocol (PDCP) and radio link control (RLC) layers, and therefore the Macro base station handles the PDCP/RLC operations for the small-cell units in addition to the Macro cell base station itself. From a physical layer/MAC layer point of view, some cross-carrier operations need to be supported in the conventional carrier aggregation operations.
For example, HARQ Acknowledge information for a secondary cell sometimes needs to be transmitted in a primary cell. Furthermore, physical control channel transmitted in a primary cell sometimes notifies the user equipment of the downlink control information for a secondary cell and vice versa in cross-carrier scheduling. That is, very tight inter-working among component carriers is required in the conventional carrier aggregation, and so the conventional carrier aggregation operations need to be operated under the single base station (the single baseband processing equipment). In other words, the conventional carrier aggregation operations between the Macro base station and the conventional Pico/Femto base station are impossible, because the Pico/Femto base station is a node different from the Macro base station. In the scenarios where remote radio heads or remote antennas which are perfectly controlled by the Macro base station are utilized, signal processing complexity of the Macro base station increases as the number of remote radio heads or remote antennas increases, because centralized control is conducted by the Macro base station. Such increasing complexity results in high cost. As a result, it is difficult to easily increase the number of the small-cell units due to high complexity and cost.
In general, the above operations, where remote radio heads or remote antennas which are perfectly controlled by the Macro base station, are called “carrier aggregation of macro cells and remote radio head cells”. The operations are described in Annex J.1 of 36.300, V a.4.0 in 3GPP specification. The operations are called “RRH CA operation” hereinafter.
The RRH CA operation has the some drawbacks. FIG. 26 illustrates a system architecture for conventional remote radio head (RRH) CA operations. In this architecture, a 2 GHz carrier (Macro2UE link) provides macro coverage and remote radio heads (RRHs) are used to improve throughput at hot spots in a 3.5 GHz carrier (RRH2UE link) Mobility is performed based on the 2 GHz carrier. In this system architecture, one common RLC layer and PDCP layer operation is conducted at Macro base stations (the base station 200A and the base station 200B) for both Macro2UE link and RRH2UE link. Very tight inter-working between Macro2UE link and RRH2UE link is also conducted from a physical layer/MAC layer point of view. This is because the remote radio head is an amplifier, and other operations including, but not limited to, coding/decoding in physical layer and MAC layer operations are conducted in the base station.
As shown in FIG. 27, however, in case that the user equipment 100 is located on the outside of the base station 200A coverage area, the user equipment 100 cannot be served by the carrier aggregation of the 2 GHz carrier and the 3.5 GHz carrier. Especially in the case where a new-type carrier, where common signals such as CRS, PSS/SSS and broadcast signals are not transmitted, is utilized in the 3.5 GHz carrier, the user equipment 100 cannot be served neither by the base station 200A, nor by the remote radio head 500A-4, because in general the user equipment 100 cannot communicate with the remote radio head 500A-4, which does not transmit such common signals, without valid connections with the base station 200A. It is noted that the new type carrier which does not contain some of common signals or broadcast signals may be called “new carrier type” or “additional carrier type” in the standardization.
The user equipment 100 can communicate with the base station 200B instead of the base station 200A in FIG. 27, but it cannot be served by the carrier aggregation of the 2 GHz carrier served by the base station 200B and the 3.5 GHz carrier served by the remote radio head 500A-4. This is because the remote radio head 500A-4 does not belong to the base station 200B, and the remote radio head 500A-4 and the base station 200B cannot have a single RLC layer and PDCP layer operation. Furthermore, the remote radio head 500A-4 and the base station 200B cannot have very tight inter-working from a physical layer and MAC layer point of view.
It clearly indicates that the conventional RRH CA operation may be cumbersome from a deployment point of view, because network operators need to align the macro cell coverage area with the RRH coverage area very accurately.
In general, the base station transmits control signals such as broadcast signals, and the user equipment communicates with the base station after receiving the control signals. That is, the user equipment cannot transmit any signals before receiving the control signals. Therefore the user equipment cannot start communications because the user equipment cannot camp on the cell, where the base station provides communication services, in idle state. That is, the user equipment cannot conduct random access procedures which are required for the initiation of the communications. This is called the principle of “transmit after receive”. This concept can prevent the user equipment from transmitting signals without any control of the network, and therefore unnecessary interference can be avoided.
However, control signals such as broadcast signals sometimes cause some backward compatibility issues. For example, network signaling, which is called “AdditionalSpectrumEmission,” is defined in TS 36.331 and in Section 6.2.4 of TS 36.101 in 3GPP specification. When the user equipment receives the network signaling, it must transmit uplink signals in order to meet additional spectrum emission requirements, which is specified in Section 6.2.4 of TS 36.101. Here, if the user equipment receives unknown network signaling in the control signals, it cannot communicate with the base station, because the user equipment may violate regulatory requirements which are related to the unknown network signaling. Especially in case that the user equipment is in idle state, the user equipment cannot camp on the cell which transmits unknown network signaling and cannot connect to the network. It means that new network signaling cannot be added after the user equipment is distributed in the market. In other words, if new network signaling is added after the user equipment is distributed in the market, a backward compatibility issue, in which the user equipment cannot communicate with the base station after that, happens.